He is a 31-year-old Franco-Tunisian man whose identity papers were found in the vehicle after the attack on France’s July 14 national holiday.
Police have not yet released the attacker’s name, but they said he lived in Nice. Other sources said previously he was already known to police for minor criminal offences.
Police shot the driver dead after he drove the truck two kilometres (1.3 miles) through a crowd along the palm-lined Promenade des Anglais.
Interior ministry spokesman Pierre-Henry Brandet said 84 people were killed and scores injured, including 18 in “critical condition”.